System and method for scrambling the phase of the carriers in a multicarrier communications system

ABSTRACT

A system and method that scrambles the phase characteristic of a carrier signal are described. The scrambling of the phase characteristic of each carrier signal includes associating a value with each carrier signal and computing a phase shift for each carrier signal based on the value associated with that carrier signal. The value is determined independently of any input bit value carried by that carrier signal. The phase shift computed for each carrier signal is combined with the phase characteristic of that carrier signal so as to substantially scramble the phase characteristic of the carrier signals. Bits of an input signal are modulated onto the carrier signals having the substantially scrambled phase characteristic to produce a transmission signal with a reduced PAR.

RELATED APPLICATION

This application claims the benefit of the filing date of copending U.S.Provisional Application Ser. No. 60/164,134, filed Nov. 9, 1999,entitled “A Method For Randomizing The Phase Of The Carriers In AMulticarrier Communications System To Reduce The Peak To Average PowerRatio Of The Transmitted Signal,” the entirety of which provisionalapplication is incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to communications systems using multicarriermodulation. More particularly, the invention relates to multicarriercommunications systems that lower the peak-to-average power ratio (PAR)of transmitted signals.

BACKGROUND OF THE INVENTION

In a conventional multicarrier communications system, transmitterscommunicate over a communication channel using multicarrier modulationor Discrete Multitone Modulation (DMT). Carrier signals (carriers) orsub-channels spaced within a usable frequency band of the communicationchannel are modulated at a symbol (i.e., block) transmission rate of thesystem. An input signal, which includes input data bits, is sent to aDMT transmitter, such as a DMT modem. The DMT transmitter typicallymodulates the phase characteristic, or phase, and amplitude of thecarrier signals using an Inverse Fast Fourier Transform (IFFT) togenerate a time domain signal, or transmission signal, that representsthe input signal. The DMT transmitter transmits the transmission signal,which is a linear combination of the multiple carriers, to a DMTreceiver over the communication channel.

The phase and amplitude of the carrier signals of DMT transmissionsignal can be considered random because the phase and amplitude resultfrom the modulation of an arbitrary sequence of input data bitscomprising the transmitted information. Therefore, under the conditionthat the modulated data bit stream is random, the DMT transmissionsignal can be approximated as having a Gaussian probabilitydistribution. A bit scrambler is often used in the DMT transmitter toscramble the input data bits before the bits are modulated to assurethat the transmitted data bits are random and, consequently, that themodulation of those bits produces a DMT transmission signal with aGaussian probability distribution.

With an appropriate allocation of transmit power levels to the carriersor sub-channels, such a system provides a desirable performance.Further, generating a transmission signal with a Gaussian probabilitydistribution is important in order to transmit a transmission signalwith a low peak-to-average ratio (PAR), or peak-to-average power ratio.The PAR of a transmission signal is the ratio of the instantaneous peakvalue (i.e., maximum magnitude) of a signal parameter (e.g., voltage,current, phase, frequency, power) to the time-averaged value of thesignal parameter. In DMT systems, the PAR of the transmitted signal isdetermined by the probability of the random transmission signal reachinga certain peak voltage during the time interval required for a certainnumber of symbols. An example of the PAR of a transmission signaltransmitted from a DMT transmitter is 14.5 dB, which is equivalent tohaving a 1E-7 probability of clipping. The PAR of a transmission signaltransmitted and received in a DMT communication system is an importantconsideration in the design of the DMT communication system because thePAR of a signal affects the communication system's total powerconsumption and component linearity requirements of the system.

If the phase of the modulated carriers is not random, then the PAR canincrease greatly. Examples of cases where the phases of the modulatedcarrier signals are not random are when bit scramblers are not used,multiple carrier signals are used to modulate the same input data bits,and the constellation maps, which are mappings of input data bits to thephase of a carrier signal, used for modulation are not random enough(i.e., a zero value for a data bit corresponds to a 90 degree phasecharacteristic of the DMT carrier signal and a one value for a data bitcorresponds to a −90 degree phase characteristic of the DMT carriersignal). An increased PAR can result in a system with high powerconsumption and/or with high probability of clipping the transmissionsignal. Thus, there remains a need for a system and method that caneffectively scramble the phase of the modulated carrier signals in orderto provide a low PAR for the transmission signal.

SUMMARY OF THE INVENTION

The present invention features a system and method that scrambles thephase characteristics of the modulated carrier signals in a transmissionsignal. In one aspect, a value is associated with each carrier signal. Aphase shift is computed for each carrier signal based on the valueassociated with that carrier signal. The value is determinedindependently of any input bit value carried by that carrier signal. Thephase shift computed for each carrier signal is combined with the phasecharacteristic of that carrier signal to substantially scramble thephase characteristics of the carrier signals.

In one embodiment, the input bit stream is modulated onto the carriersignals having the substantially scrambled phase characteristic toproduce a transmission signal with a reduced peak-to-average power ratio(PAR). The value is derived from a predetermined parameter, such as arandom number generator, a carrier number, a DMT symbol count, asuperframe count, and a hyperframe count. In another embodiment, apredetermined transmission signal is transmitted when the amplitude ofthe transmission signal exceeds a certain level.

In another aspect, the invention features a method wherein a value isassociated with each carrier signal. The value is determinedindependently of any input bit value carried by that carrier signal. Aphase shift for each carrier signal is computed based on the valueassociated with that carrier signal. The transmission signal isdemodulated using the phase shift computed for each carrier signal.

In another aspect, the invention features a system comprising a phasescrambler that computes a phase shift for each carrier signal based on avalue associated with that carrier signal. The phase scrambler alsocombines the phase shift computed for each carrier signal with the phasecharacteristic of that carrier signal to substantially scramble thephase characteristic of the carrier signals. In one embodiment, amodulator, in communication with the phase scrambler, modulates bits ofan input signal onto the carrier signals having the substantiallyscrambled phase characteristics to produce a transmission signal with areduced PAR.

DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.The advantages of the invention described above, as well as furtheradvantages of the invention, may be better understood by reference tothe following description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of an embodiment of a digital subscriber linecommunications system including a DMT (discrete multitone modulation)transceiver, in communication with a remote transceiver, having a phasescrambler for substantially scrambling the phase characteristics ofcarrier signals; and

FIG. 2 is a flow diagram of an embodiment of a process for scramblingthe phase characteristics of the carrier signals in a transmissionsignal.

DETAILED DESCRIPTION

FIG. 1 shows a digital subscriber line (DSL) communication system 2including a discrete multitone (DMT) transceiver 10 in communicationwith a remote transceiver 14 over a communication channel 18 using atransmission signal 38 having a plurality of carrier signals. The DMTtransceiver 10 includes a DMT transmitter 22 and a DMT receiver 26. Theremote transceiver 14 includes a transmitter 30 and a receiver 34.Although described with respect to discrete multitone modulation, theprinciples of the invention apply also to other types of multicarriermodulation, such as, but not limited to, orthogonally multiplexedquadrature amplitude modulation (OQAM), discrete wavelet multitone(DWMT) modulation, and orthogonal frequency division multiplexing(OFDM).

The communication channel 18 provides a downstream transmission pathfrom the DMT transmitter 22 to the remote receiver 34, and an upstreamtransmission path from the remote transmitter 30 to the DMT receiver 26.In one embodiment, the communication channel 18 is a pair of twistedwires of a telephone subscriber line. In other embodiments, thecommunication channel 18 can be a fiber optic wire, a quad cable,consisting of two pairs of twisted wires, or a quad cable that is one ofa star quad cable, a Dieselhorst-Martin quad cable, and the like. In awireless communication system wherein the transceivers 10, 14 arewireless modems, the communication channel 18 is the air through whichthe transmission signal 38 travels between the transceivers 10, 14.

By way of example, the DMT transmitter 22 shown in FIG. 1 includes aquadrature amplitude modulation (QAM) encoder 42, a modulator 46, a bitallocation table (BAT) 44, and a phase scrambler 66. The DMT transmitter22 can also include a bit scrambler 74, as described further below. Theremote transmitter 30 of the remote transceiver 14 comprises equivalentcomponents as the DMT transmitter 22. Although this embodiment specifiesa detailed description of the DMT transmitter 22, the inventive conceptsapply also to the receivers 34, 36 which have similar components to thatof the DMT transmitter 22, but perform inverse functions in a reverseorder.

The QAM encoder 42 has a single input for receiving an input serial databit stream 54 and multiple parallel outputs to transmit QAM symbols 58generated by the QAM encoder 42 from the bit stream 54. In general, theQAM encoder 42 maps the input serial bit-stream 54 in the time domaininto parallel QAM symbols 58 in the frequency domain. In particular, theQAM encoder 42 maps the input serial data bit stream 54 into N parallelquadrature amplitude modulation (QAM) constellation points 58, or QAMsymbols 58, where N represents the number of carrier signals generatedby the modulator 46. The BAT 44 is in communication with the QAM encoder42 to specify the number of bits carried by each carrier signal. The QAMsymbols 58 represent the amplitude and the phase characteristic of eachcarrier signal.

The modulator 46 provides functionality associated with the DMTmodulation and transforms the QAM symbols 58 into DMT symbols 70 eachcomprised of a plurality of time-domain samples. The modulator 46modulates each carrier signal with a different QAM symbol 58. As aresult of this modulation, carrier signals have phase and amplitudecharacteristics based on the QAM symbol 58 and therefore based on theinput-bit stream 54. In particular, the modulator 46 uses an inversefast Fourier transform (IFFT) to change the QAM symbols 58 into atransmission signal 38 comprised of a sequence of DMT symbols 70. Themodulator 46 changes the QAM symbols 58 into DMT symbols 70 throughmodulation of the carrier signals. In another embodiment, the modulator46 uses the inverse discrete Fourier transform (IDFT) to change the QAMsymbols 58 into DMT symbols 70. In one embodiment, a pilot tone isincluded in the transmission signal 38 to provide a reference signal forcoherent demodulation of the carrier signals in the remote receiver 34during reception of the transmission signal 38.

The modulator 46 also includes a phase scrambler 66 that combines aphase shift computed for each QAM-modulated carrier signal with thephase characteristic of that carrier signal. Combining phase shifts withphase characteristics, in accordance with the principles of theinvention, substantially scrambles the phase characteristics of thecarrier signals in the transmission signal 38. By scrambling the phasecharacteristics of the carrier signals, the resulting transmissionsignal 38 has a substantially minimized peak-to-average (PAR) powerratio. The phase scrambler 66 can be part of or external to themodulator 46. Other embodiments of the phase scrambler 66 include, butare not limited to, a software program that is stored in local memoryand is executed on the modulator 46, a digital signal processor (DSP)capable of performing mathematical functions and algorithms, and thelike. The remote receiver 34 similarly includes a phase scrambler 66′for use when demodulating carrier signals that have had their phasecharacteristics adjusted by the phase scrambler 66 of the DMTtransceiver 10.

To compute a phase shift for each carrier signal, the phase scrambler 66associates one or more values with that carrier signal. The phasescrambler 66 determines each value for a carrier signal independently ofthe QAM symbols 58, and, therefore, independently of the bit value(s)modulated onto the carrier signal. The actual value(s) that the phasescrambler 66 associates with each carrier signal can be derived from oneor more predefined parameters, such as a pseudo-random number generator(pseudo-RNG), a DMT carrier number, a DMT symbol count, a DMT superframecount, a DMT hyperframe count, and the like, as described in more detailbelow. Irrespective of the technique used to produce each value, thesame technique is used by the DMT transmitter 22 and the remote receiver34 so that the value associated with a given carrier signal is known atboth ends of the communication channel 18.

The phase scrambler 66 then solves a predetermined equation to compute aphase shift for the carrier signal, using the value(s) associated withthat carrier signal as input that effects the output of the equation.Any equation suitable for computing phase shifts can be used to computethe phase shifts. When the equation is independent of the bit values ofthe input serial bit stream 54, the computed phase shifts are alsoindependent of such bit values.

In one embodiment (shown in phantom), the DMT transmitter 22 includes abit scrambler 74, which receives the input serial bit stream 54 andoutputs data bits 76 that are substantially scrambled. The substantiallyscrambled bits 76 are then passed to the QAM encoder 42. When the bitscrambler 74 is included in the DMT transmitter 22, the operation of thephase scrambler 66 further assures that the transmission signal 38 has aGaussian probability distribution and, therefore, a substantiallyminimized PAR.

FIG. 2 shows embodiments of a process used by the DMT transmitter 22 foradjusting the phase characteristic of each carrier signal and combiningthese carrier signals to produce the transmission signal 38. The DMTtransmitter 22 generates (step 100) a value that is associated with acarrier signal. Because the value is being used to alter the phasecharacteristics of the carrier signal, both the DMT transmitter 22 andthe remote receiver 34 must recognize the value as being associated withthe carrier signal. Either the DMT transmitter 22 and the remotereceiver 34 independently derive the associated value, or one informsthe other of the associated value. For example, in one embodiment theDMT transmitter 22 can derive the value from a pseudo-RNG and thentransmit the generated value to the remote receiver 34. In anotherembodiment, the remote receiver 34 similarly derives the value from thesame pseudo-RNG and the same seed as used by the transmitter (i.e., thetransmitter pseudo-RNG produces the same series of random numbers as thereceiver pseudo-RNG).

As another example, the DMT transmitter 22 and the remote receiver 34can each maintain a symbol counter for counting DMT symbols. The DMTtransmitter 22 increments its symbol counter upon transmitting a DMTsymbol; the remote receiver 34 upon receipt. Thus, when the DMTtransmitter 22 and the remote receiver 34 both use the symbol count as avalue for computing phase shifts, both the DMT transmitter 22 and remotereceiver 34 “know” that the value is associated with a particular DMTsymbol and with each carrier signal of that DMT symbol.

Values can also be derived from other types of predefined parameters.For example, if the predefined parameter is the DMT carrier number, thenthe value associated with a particular carrier signal is the carriernumber of that signal within the DMT symbol. The number of a carriersignal represents the location of the frequency of the carrier signalrelative to the frequency of other carrier signals within a DMT symbol.For example, in one embodiment the DSL communication system 2 provides256 carrier signals, each separated by a frequency of 4.3125 kHz andspanning the frequency bandwidth from 0 kHz to 1104 kHz. The DMTtransmitter 22 numbers the carrier signals from 0 to 255. Therefore,“DMT carrier number 50” represents the 51st DMT carrier signal which islocated at the frequency of 215.625 kHz (i.e., 51×4.3125 kHz).

Again, the DMT transmitter 22 and the remote receiver 34 can know thevalue that is associated with the carrier signal because both the DMTtransmitter 22 and the remote receiver 34 use the same predefinedparameter (here, the DMT carrier number) to make the value-carriersignal association. In other embodiments (as exemplified above with thetransmitter pseudo-RNG), the DMT transmitter 22 can transmit the valueto the remote receiver 34 (or vice versa) over the communication channel18.

In other embodiments, other predefined parameters can be used inconjunction with the symbol count. One example of such a predefinedparameter is the superframe count that increments by one every 69 DMTsymbols. One exemplary implementation that achieves the superframecounter is to perform a modulo 68 operation on the symbol count. Asanother example, the DMT transmitter 22 can maintain a hyperframecounter for counting hyperframes. An exemplary implementation of thehyperframe count is to perform a modulo 255 operation on the superframecount. Thus, the hyperframe count increments by one each time thesuperframe count reaches 255.

Accordingly, it is seen that some predefined parameters produce valuesthat vary from carrier signal to carrier signal. For example, when thepredefined parameter is the DMT carrier number, values vary based on thefrequency of the carrier signal. As another example, the pseudo-RNGgenerates a new random value for each carrier signal.

Other predefined parameters produce values that vary from DMT symbol 70to DMT symbol 70. For example, when the predefined parameter is thesymbol count, the superframe count, or hyperframe count, values varybased on the numerical position of the DMT symbol 70 within a sequenceof symbols, superframes, or hyperframes. Predefined parameters such asthe pseudo-RNG, symbol count, superframe count, and superframe can alsobe understood to be parameters that vary values over time. Any one orcombination of the predefined parameters can provide values for input tothe equation that computes a phase shift for a given carrier signal.

In one embodiment, the phase scrambling is used to avoid clipping of thetransmission signal 38 on a DMT symbol 70 by DMT symbol 70 basis. Inthis embodiment, the DMT transmitter 22 uses a value based on apredefined parameter that varies over time, such as the symbol count, tocompute the phase shift. It is to be understood that other types ofpredefined parameters that vary the values associated with carriersignals can be used to practice the principles of the invention. Asdescribed above, the transceivers 10, 14 may communicate (step 110) thevalues to synchronize their use in modulating and demodulating thecarrier signals.

The DMT transmitter 22 then computes (step 115) the phase shift that isused to adjust the phase characteristic of each carrier signal. Theamount of the phase shift combined with the phase characteristic of eachQAM-modulated carrier signal depends upon the equation used and the oneor more values associated with that carrier signal.

The DMT transmitter 22 then combines (step 120) the phase shift computedfor each carrier signal with the phase characteristic of that carriersignal. By scrambling the phase characteristics of the carrier signals,the phase scrambler 66 reduces (with respect to unscrambled phasecharacteristics) the combined PAR of the plurality of carrier signalsand, consequently, the transmission signal 38. The following three phaseshifting examples, PS #1-PS #3, illustrate methods used by the phasescrambler 66 to combine a computed phase shift to the phasecharacteristic of each carrier signal.

PHASE SHIFTING EXAMPLE #1

Phase shifting example #1 (PS #1) corresponds to adjusting the phasecharacteristic of the QAM-modulated carrier signal associated with acarrier number N by

${N \times \frac{\pi}{3}},$modulo (mod) 2π. In this example, a carrier signal having a carriernumber N equal to 50 has a phase shift added to the phase characteristicof that carrier signal equal to

${50 \times \frac{\pi}{3}\left( {{mod}{\mspace{11mu}\;}2\pi} \right)} = {\frac{2}{3}{\pi.}}$The carrier signal with a carrier number N equal to 51 has a phase shiftadded to the phase characteristic of that carrier signal equal to

${51 \times \frac{\pi}{3}\left( {{mod}\mspace{14mu} 2\pi} \right)} = {\pi.}$The carrier signal with the carrier number N equal to 0 has no phaseshift added to the phase characteristic of that carrier signal.

PHASE SHIFTING EXAMPLE #2

Phase shifting example #2 (PS #2) corresponds to adjusting the phasecharacteristic of the QAM-modulated carrier signal associated with acarrier number N by

${\left( {N + M} \right) \times \frac{\pi}{4}},$mod 2π, where M is the symbol count. In this example, a carrier signalhaving a carrier number N equal to 50 on DMT symbol count M equal to 8has a phase shift added to the phase characteristic of that carriersignal equal to

${\left( {50 + 8} \right) \times \frac{\pi}{4}\left( {{mod}\mspace{14mu} 2\pi} \right)} = {\frac{\pi}{2}.}$The carrier signal with the same carrier number N equal to 50 on thenext DMT symbol count M equal to 9 has a phase shift added to the phasecharacteristic of that carrier signal equal to

${\left( {50 + 9} \right) \times \frac{\pi}{4}\left( {{mod}\mspace{14mu} 2\pi} \right)} = {\frac{3\pi}{4}.}$

PHASE SHIFTING EXAMPLE #3

Phase shifting example #3 (PS #3) corresponds to adjusting the phasecharacteristic of the QAM-modulated carrier signal associated with acarrier number N by

${\left( X_{N} \right) \times \frac{\pi}{6}},$mod 2π, where X_(N) is an array of N pseudo-random numbers. In thisexample, a carrier signal having a carrier number N equal to 5 and X_(N)equal to [3, 8, 1, 4, 9, 5, . . . ] has a phase shift added to the phasecharacteristic of the carrier signal that is equal to

${(9) \times \frac{\pi}{6}\left( {{mod}\mspace{14mu} 2\pi} \right)} = \frac{3\pi}{2}$(Note that 9 is the 5^(th) value in X_(N).) The carrier signal with acarrier number N equal to 6 has a phase shift added to the phasecharacteristic of the carrier signal equal to

${(5) \times \frac{\pi}{6}\left( {{mod}\mspace{14mu} 2\pi} \right)} = {\frac{5\pi}{6}.}$

It is to be understood that additional and/or different phase shiftingtechniques can be used by the phase scrambler 66, and that PS #1, #2,and #3 are merely illustrative examples of the principles of theinvention. The DMT transmitter 22 then combines (step 130) the carriersignals to form the transmission signal 38. If the transmission signalis not clipped, as described below, the DMT transmitter 22 consequentlytransmits (step 160) the transmission signal 38 to the remote receiver34.

Clipping of Transmission Signals

A transmission signal 38 that has high peak values of voltage (i.e., ahigh PAR) can induce non-linear distortion in the DMT transmitter 22 andthe communication channel 18. One form of this non-linear distortion ofthe transmission signal 38 that may occur is the limitation of theamplitude of the transmission signal 38 (i.e., clipping). For example, aparticular DMT symbol 70 clips in the time domain when one or more timedomain samples in that DMT symbol 70 are larger than the maximum alloweddigital value for the DMT symbols 70. In multicarrier communicationsystems when clipping occurs, the transmission signal 38 does notaccurately represent the input serial data bit signal 54.

In one embodiment, the DSL communication system 2 avoids the clipping ofthe transmission signal 38 on a DMT symbol 70 by DMT symbol 70 basis.The DMT transmitter 22 detects (step 140) the clipping of thetransmission signal 38. If a particular DMT symbol 70 clips in the timedomain to produce a clipped transmission signal 38, the DMT transmitter22 substitutes (step 150) a predefined transmission signal 78 for theclipped transmission signal 38.

The predefined transmission signal 78 has the same duration as a DMTsymbol 70 (e.g., 250 ms) in order to maintain symbol timing between theDMT transmitter 22 and the remote receiver 34. The predefinedtransmission signal 78 is not based on (i.e., independent of) themodulated input data bit stream 54; it is a bit value pattern that isrecognized by the remote receiver 34 as a substituted signal. In oneembodiment, the predefined transmission signal 78 is a knownpseudo-random sequence pattern that is easily detected by the remotereceiver 34. In another embodiment, the predefined transmission signal78 is an “all zeros” signal, which is a zero voltage signal produced atthe DMT transmitter 22 output (i.e., zero volts modulated on all thecarrier signals). In addition to easy detection by the remote receiver34, the zero voltage signal reduces the power consumption of the DMTtransmitter 22 when delivered by the DMT transmitter 22. Further, apilot tone is included in the predefined transmission signal 78 toprovide a reference signal for coherent demodulation of the carriersignals in the remote receiver 34 during reception of the predefinedtransmission signal 78.

After the remote receiver 34 receives the transmission signal 38, theremote receiver 34 determines if the transmission signal 38 isequivalent to the predefined transmission signal 78. In one embodiment,when the remote receiver 34 identifies the predefined transmissionsignal 78, the remote receiver 34 ignores (i.e., discards) thepredefined transmission signal 78.

Following the transmission of the predefined transmission signal 78, thephase scrambler 66 shifts (step 120) the phase characteristic of theQAM-modulated carrier signals (based on one of the predefined parametersthat varies over time). For example, consider that a set of QAM symbols58 produces a DMT symbol 70 comprising a plurality of time domainsamples, and that one of the time domain samples is larger than themaximum allowed digital value for the DMT symbol 70. Therefore, becausethe transmission signal 38 would be clipped when sent to the remotereceiver 34, the DMT transmitter 22 sends the predefined transmissionsignal 78 instead.

After transmission of the predefined transmission signal 78, the DMTtransmitter 22 again attempts to send the same bit values that producedthe clipped transmission signal 38 in a subsequent DMT symbol 70′.Because the generation of phase shifts in this embodiment is based onvalues that vary over time, the phase shifts computed for the subsequentDMT symbol 70′ are different than those that were previously computedfor the DMT symbol 70 with the clipped time domain sample. Thesedifferent phase shifts are combined to the phase characteristics of themodulated carrier signals to produce carrier signals of the subsequentDMT symbol 70′ with different phase characteristics than the carriersignals of the DMT symbol 70 with the clipped time domain sample.

DMT communication systems 2 infrequently produce transmission signals 38that clip (e.g., approximately one clip every 10⁷ time domain samples70). However, if the subsequent DMT symbol 70′ includes a time domainsample that clips, then the predefined transmission signal 78 is againtransmitted (step 150) to the remote receiver 34 instead of the clippedtransmission signal 38. The clipping time domain sample may be on thesame or on a different carrier signal than the previously clipped DMTsymbol 70. The DMT transmitter 22 repeats the transmission of thepredefined transmission signal 78 until the DMT transmitter 22 producesa subsequent DMT symbol 70′ that is not clipped. When the DMTtransmitter 22 produces a DMT symbol 70′ that is not clipped, the DTMtransmitter 22 transmits (step 160) the transmission signal 38 to theremote receiver 34. The probability of a DMT symbol 70 producing atransmission signal 38 that clips in the time domain depends on the PARof the transmission signal 38.

For example, the following phase shifting example, PST #4, illustratesthe method used by the phase scrambler 66 to combine a different phaseshift to the phase characteristic of each carrier signal to avoid theclipping of the transmission signal 38.

PHASE SHIFTING EXAMPLE #4

Phase shifting example #4 (PS #4) corresponds to adjusting the phasecharacteristic of the carrier signal associated with a carrier number Nby

${\frac{\pi}{3} \times \left( {M + N} \right)},$mod 2π, where M is the DMT symbol count. In this example, if the DMTsymbol 70 clips when the DMT symbol count M equals 5, the predefinedtransmission signal 78 is transmitted instead of the current clippedtransmission signal 38. On the following DMT symbol period, the DMTcount M equals 6, thereby causing a different set of time domain samplesto be generated for the subsequent DMT symbol 70′, although the QAMsymbols 58 used to produce both DMT symbols 70, 70′ are the same.

If this different set of time domain samples (and consequently thetransmission signal 38) is not clipped, the DMT transmitter 22 sends thetransmission signal 38. If one of the time domain samples in thedifferent set of time domain samples 70 (and consequently thetransmission signal 38) is clipped, then the DMT transmitter 22 sendsthe predefined transmission signal 78 again. The process continues untila DMT symbol 70 is produced without a time domain sample 70 that isclipped. In one embodiment, the transmitter 22 stops attempting toproduce a non-clipped DMT symbol 70′ for the particular set of QAMsymbols 58 after generating a predetermined number of clipped DMTsymbols 70′. At that moment, the transmitter 22 can transmit the mostrecently produced clipped DMT symbol 70′ or the predeterminedtransmission signal 78.

The PAR of the DSL communication system 2 is reduced because thepredefined transmission signal 78 is sent instead of the transmissionsignal 38 when the DMT symbol 70 clips. For example, a DMT communicationsystem 2 that normally has a clipping probability of 10⁻⁷ for the timedomain transmission signal 38 can therefore operate with a 10⁻⁵probability of clipping and a lower PAR equal to 12.8 dB (as compared to14.5 dB). When operating at a 10⁻⁵ probability of clipping, assuming aDMT symbol 70 has 512 time-domain samples 70, the DMT transmitter 22experiences one clipped DMT symbol 70 out of every

$\frac{10^{5}}{512},$or 195 DMT symbols 70. This results in the predefined (non-datacarrying) transmission signal 78 being transmitted, on average, onceevery 195 DMT symbols. Although increasing the probability of clippingto 10⁻⁵ results in approximately a 0.5% (1/195) decrease in throughput,the PAR of the transmission signal 38 is reduced by 1.7 dB, whichreduces transmitter complexity in the form of power consumption andcomponent linearity.

While the invention has been shown and described with reference tospecific preferred embodiments, it should be understood by those skilledin the art that various changes in form and detail may be made thereinwithout departing from the spirit and scope of the invention as definedby the following claims. For example, although the specification usesDSL to describe the invention, it is to be understood that various formof DSL can be used, e.g., ADSL, VDSL, SDSL, HDSL, HDSL2, or SHDSL. It isalso to be understood that the principles of the invention apply tovarious types of applications transported over DSL systems (e.g.,telecommuting, video conferencing, high speed Internet access, video-ondemand).

1. In a multicarrier modulation system including a first transceiver incommunication with a second transceiver using a transmission signalhaving a plurality of carrier signals for modulating an input bitstream, each carrier signal having a phase characteristic associatedwith the input bit stream, a method for scrambling the phasecharacteristics of the carrier signals comprising: associating eachcarrier signal with a value determined independently of any input bitvalue carried by that carrier signal; computing a phase shift for eachcarrier signal based on the value associated with that carrier signal;and combining the phase shift computed for each carrier signal with thephase characteristic of that carrier signal so as to substantiallyscramble the phase characteristics of the plurality of carrier signals.2. The method of claim 1, further comprising modulating bits of theinput bit stream onto the carrier signals having the substantiallyscrambled phase characteristics to produce a transmission signal with areduced peak-to-average power ratio (PAR).
 3. The method of claim 1,further comprising independently deriving the value associated with eachcarrier signal at each transceiver.
 4. The method of claim 1, furthercomprising transmitting the value associated with each carrier signalfrom one transceiver to the other transceiver.
 5. The method of claim 1,further comprising maintaining synchronization between the transceiversusing the value associated with each carrier signal.